The invention relates to a laser crystal device for short pulse lasers, comprising a container whose interior is sealed in relation to the environment and which contains a technically pure atmosphere, and which includes windows attached on side walls for the passage of laser radiation, which passes through a laser crystal in operation, the windows being situated tilted by the Brewster's angle to the beam path of the laser radiation, and a mounting for the laser crystal being attached in the interior of the container. Such a laser crystal device is known from U.S. Pat. No. 6,002,697 A.
Furthermore, the invention relates to laser oscillators having such a laser crystal device.
Modern laser oscillators for the generation of short laser pulses have a high peak power because of the short pulse duration, notwithstanding the low average power. Thus, in the case of mode-coupled femtosecond laser oscillators, for example, having a pulse duration less than 100 fs, a degradation of the crystal surface can be the result due to the high intensity of the laser radiation, i.e., due to the high peak power, even if the fluence at the crystal surface is generally well below the destruction threshold of the crystal. Such damage of the crystal surface results in disturbances in the laser operation and in a higher absorption of the crystal and, as a consequence, in a destruction of the laser crystal.
The impairment of the crystal surface is a function of the atmosphere surrounding the crystal and the intensity of the laser radiation. In the case of a pure atmosphere and/or a low laser intensity, no degradation of the laser crystal occurs. On the other hand, it has been shown that degradation processes can occur even in clean rooms, for example, if electronic devices outgas in proximity to the laser device.
A diode-pumped laser having frequency multiplier stages is known from above-mentioned U.S. Pat. No. 6,002,697 A, nonlinear laser crystals being provided in the latter, which are situated in sealed containers in order to prevent penetration of moisture or contaminants from the outside. In particular, it is a concern here to prevent impairments of the laser crystal by absorbing and releasing moisture upon cooling and heating and it is additionally also provided for this purpose that an inert gas atmosphere or dry air is to be provided in the interior of the housing or container, or the container interior is to be evacuated. The windows for the entry or exit of the laser radiation, which are situated at the Brewster's angle to the laser radiation, are situated tilted to one another on diametrically opposing sides of the container, which is generally cuboid. The design of this laser crystal device is thus comparatively complex and voluminous. The continuous monitoring and flushing of the inert gas atmosphere in the container interior described hereafter, notwithstanding the expenditure connected thereto during operation of the associated laser apparatus, has also proven to be disadvantageous for the laser operation.
A laser crystal device for a laser amplifier is disclosed in EP 1 034 584 B, in which the laser crystal is housed in a tightly encapsulated container, having laser beam coupling windows on separate pipe sockets. The interior of the container is evacuated here and/or kept dry using a desiccant. The background of these measures is that the laser crystal is to be strongly cooled with the aid of Peltier elements, in order to achieve a high efficiency of the laser amplifier, which is assigned to the laser crystal, the baking of condensed water or ice on the crystal surface being prevented by the evacuation or drying of the container interior.
However, without such strong cooling in laser oscillators, degradation of the crystal surface can also occur if the environment of the laser crystal is not provided as a “pure” atmosphere.
Furthermore, it has been shown that the windows provided on the containers, via which the laser light reaches the crystal and is coupled out, may also be subject to a degradation, whereby the operation of the laser device to which the laser crystal belongs can also be impaired. This is significant in particular in the case of a laser amplifier, as is concerned in EP 1 034 584 B, and in which particularly high peak powers occur in comparison to a short pulse laser apparatus; accordingly, the laser beam windows situated at the Brewster's angle are externally attached on separate pipe sockets in this known laser crystal device for a laser amplifier, as noted, in order to thus achieve the greatest possible distance to the laser crystal, in the magnitude of 8 to 10 cm. A relatively large beam cross-section is thus obtained in the area of the Brewster's windows, in order to thus obtain a comparatively low peak intensity at this position. A reflection on the windows is counteracted per se by the arrangement of the windows at the Brewster's angle (which is known to be a function, inter alia, of the wavelength or frequency of the radiation).